The present invention relates to cemented carbides, formed from molybdenum monocarbide-tungsten monocarbide solid solutions, and specifically, to a new and improved process for forming said solid solutions.
Cemented carbides are well known for their unique combination of hardness, strength and abrasion resistance and are, accordingly, extensively used for such industrial applications as cutting tools, drawing dies, wear parts, and other applications requiring these properties. They are produced by powder metallurgy techniques involving the liquid phase sintering of one or more refractory carbides of Groups IV, V and VI of the Periodic Table with one or more of the iron group metals. The iron group metal exists as a matrix or binder in the sintered alloy and acts to bond or cement the refractory carbide particles together.
Tungsten carbide (WC) compositions, the tungsten carbide, having a hexagonal crystal structure, has been preferred by those skilled in the art because of their high strength and good abrasion resistance. Unfortunately, however, tungsten is a relatively rare metal, and thus, the use thereof in a tool greatly increases the cost of the tool. Accordingly, it has become desirable to substitute another less expensive metal for tungsten, while still maintaining the high abrasion resistance and toughness exhibited by tungsten carbide.
One metal which has been contemplated as a viable substitute for tungsten is molybdenum. Such a substitute has been suggested for several reasons. First, molybdenum is adjacent tungsten in the Periodic Table, and sometimes forms compounds with other elements which are analogous to similar tungsten compounds and which have similar physical properties. Second, molybdenum is a relatively abundant and inexpensive metal.
Numerous attempts have been made to synthesize the molybdenum carbide (Mo C) analog to tungsten carbide (WC), but for the most part, these attempts have failed to yield homogeneous and defined products so that until fairly recently, even the existence of the hexagonal molybdenum monocarbide remained in question. It is known that WC and Mo C form a series of continuous solid solutions based upon the WC hexagonal crystal lattice and efforts have been made to stabilize Mo C by the addition of WC. However, the difficulty in preparing single phase monocarbides, and the experienced instability of the solid solutions in the presence of cobalt at temperatures in the range of 1350.degree. to 1500.degree. C. has discouraged attempts to fabricate cemented carbides containing Mo C. Until fairly recently, it was accepted in the prior art that not more than 1-2 percent of the tungsten in WC could be exchanged with molybdenum, and that the solid solution (Mo,W)C or Mo C did not exist in the desired temperature range of 1200.degree. to 1900.degree. C.
It has now been found that Mo C and single phased (Mo,W)C solid solutions having greater than 1-2 percent Mo can be prepared under certain conditions. Such a teaching is disclosed in U.S. Pat. No. 4,049,380 to Yih etal., issued Sept. 20, 1977, and entitled "Cemented Carbides Containing Hexagonal Molybdenum". Said reference, the teachings of which are incorporated herein by reference, discloses that Mo C or the solid solution (Mo,W)C may be prepared by heating a blended mixture of the desired gross components, which may be powdered molybdenum and tungsten metal and graphite, or a mixture of Mo.sub.2 C, WC and graphite in the presence of approximately 0.5 to 1.0 percent by weight of nickel or cobalt to a temperature at which nucleation of the hexagonal Mo C phase begins. More particularly, the reference teaches first forming the cubic solid solutions (Mo,W).sub.3 C.sub.2 or (Mo,W)C.sub.1-x by heating the desired composition to temperatures in excess of 1700.degree. C., preferably in excess of 2000.degree. C. and then to lower the temperature to within the stability domain of the hexagonal Mo C or (Mo,W)C solid solution, and holding at this temperature for several hours until the formation of the monocarbide is complete.
It is an object of the present invention to provide a new and improved process for forming molybdenum monocarbidetungsten monocarbide solid solutions, and specifically, to provide such a process which can be performed at temperatures significantly lower than the 1700.degree.-2000.degree. C. and excess temperature required in the prior art.